docs: def _to_mongo_query(query):
          """
          Convert the query received by the Sacred Web API to a MongoDB query.
  
          Takes a query in format
          {"type": "and", "filters": [
           {"field": "host.hostname", "operator": "==", "value": "ntbacer"},
           {"type": "or", "filters": [
              {"field": "result", "operator": "==", "value": 2403.52},
              {"field": "host.python_version", "operator": "==", "value":"3.5.2"}
              ]}]}
          and returns an appropriate MongoDB Query.
          :param query: A query in the Sacred Web API format.
          :return: Mongo Query.
          """
          mongo_query = []
          for clause in query["filters"]:
              if clause.get("type") is None:
                  mongo_clause = MongoRunDAO. \
                      _simple_clause_to_query(clause)
              else:
                  # It's a subclause
                  mongo_clause = MongoRunDAO._to_mongo_query(clause)
              mongo_query.append(mongo_clause)
  
          if len(mongo_query) == 0:
              return {}
          if query["type"] == "and":
              return {"$and": mongo_query}
          elif query["type"] == "or":
              return {"$or": mongo_query}
          else:
              raise ValueError("Unexpected query type %s" % query.get("type"))